Chemical Engineering April 2021 - 52

FIGURE 4. An interior view of the rotary dryer with
flights is shown here
tially damage the unit. By opting for
a process audit, the path to issue
resolution is expedited.
Design considerations
In addition to operational considerations,
there are several aspects of
the dryer that should be considered
during the initial design stages to
ensure the dryer is suitable for the
intended application and will provide
long-term reliability.
Choose a dryer suited to its intended
level of duty. One of the
most common mistakes made in
acquiring or designing a rotary dryer
is the application of a one-size-fitsall
design. The applications in which
rotary dryers are employed vary substantially
in their level of duty, and in
the specific challenges presented by
each job. As such, a one-size-fits-all
solution almost always falls short.
Dryers designed for use in the biomass
industry, for example, would
quickly
characteristics of the material to be
dried, in combination with drum rotation
speed, slope, air velocity and
retention time. Flights not designed
around such criteria can reduce efficiency,
drive up fuel costs, promote
buildup, and lead to other problems.
Flights can be customized in both
design and pattern, with many dryers
incorporating multiple flight designs
to accommodate the changes
that occur in the material as it moves
through the dryer.
Consider a combustion chamber.
The combustion chamber is a static
vessel affixed between the burner
and drum that serves to house the
combustion reaction (Figure 5). Rotary
dryers are available with or without
a combustion chamber. In some
cases, they may be required to reach
the precise drying conditions a given
job demands, or to maintain product
quality, while in other cases, they are
not required, but could bring advantages
to the system.
fail under the demanding
conditions of the mining industry.
For long-term reliable operation,
dryers must be designed around the
level of duty required by the intended
application. This often means selecting
different materials of construction,
shell thicknesses, mechanical
components, and seals for heavyduty
applications. The materials of
construction, along with stringent
quality standards in fabrication, are
essential to acquiring a dryer that
achieves a long lifetime and experiences
minimal issues.
Optimize flight design. As with the
overall design and construction of
the rotary dryer, taking the time to
optimize the flight design during the
initial design process is also important
in achieving an efficient and reliable
drying solution (Figure 4).
Flights contribute significantly to
drying efficiency, but only if they are
designed to work with the unique
52
When the material being dried
is heat-sensitive or could be damaged
in some way by exposure to a
flame, combustion chambers are effective
at preventing flame contact.
Preventing contact with the flame
is also useful with materials that
could form pollutants if exposed to a
flame, which would increase the requirements
for an exhaust-gas treatment
system. For these reasons,
potash dryers, for example, usually
incorporate a combustion chamber,
because potash will not only break
down under exposure to a flame,
resulting in inferior product, but will
also produce undesirable pollutants
if allowed to come into contact with
the flame.
The use of a dilution air fan can
also improve process control, by allowing
the gas temperature entering
the chamber to be more readily
controlled. This improves overall efficiency
and promotes a more complete
combustion of the fuel.
Further, since there is no contact
between the flame and the drum shell,
dryers with a combustion chamber
often last longer than their counterparts
without combustion chambers.
Various combustion-chamber
designs exist to optimize them for
specific requirements. They are typically
lined with refractory and may be
single- or double-shelled. The chamFIGURE
5. The burner feeds into a combustion
chamber attached to a rotary dryer
bers can also be inline, right-angled,
or placed on a floor above or below
with a 180-deg elbow.
Operators struggling with either
overdried product, product that has
been deleteriously affected by the
dryer, or problems with controlling the
product outlet moisture, may benefit
from the retrofit of a combustion
chamber onto their existing dryer.
Concluding remarks
Rotary dryers are a substantial investment
in any process. By following
best practices in dryer operation
and maintenance, that investment
can be maximized for reliable longterm
processing. The best practices
mentioned here, along with choosing
a dryer that is well suited to its
intended application, including optimized
flight design and the use of a
combustion chamber where applicable,
will help plant managers protect
their investment and be confident
that their dryer will provide reliable
performance for the long term. 
Edited by Scott Jenkins
Authors
Shane Le Capitaine is a
process sales engineer at FEECO
International Inc. (3913 Algoma
Rd.,
Green Bay, WI 54311;
Phone: 1-800-373-9347; Email:
slecapitaine@feeco.com), where
he specializes in bulk solids drying,
fertilizer granulation, and rotary
drum applications. Le Capitaine
received a B.S.Ch.E. from
the University of Wisconsin-Madison and has over 20
years of experience in his field.
Carrie Carlson is a technical
writer who has been with
FEECO (same address as above;
Email: ccarlson@feeco.com) for
a decade. She works closely with
engineers and process experts to
turn complex ideas into easy-tounderstand
literature.
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